Heat-sensitive mimeograph stencils (hereinafter referred to as "stencils" for short) are known which comprise a thermoplastic film such as acrylonitrile-based film, polyester film, vinylidene chloride film or the like and a porous support such as a tissue paper mainly comprising natural fibers or synthetic fibers, a non-woven fabric or a woven fabric, which is adhered to the above-mentioned thermoplastic film. For example, Japanese Laid-open Patent Application (Kokai) No. 51-2512 discloses a stencil comprising an acrylonitrile-based film and an ink-permeable support adhered to the film; Japanese Laid-open Patent Application (Kokai) No. 51-2513 discloses a stencil comprising an oriented polyethylene terephthalate film and an ink-permeable support adhered to the film; and Japanese Laid-open Patent Application (Kokai) No. 57-182495 discloses a stencil comprising a polyester film and a porous tissue paper or a mesh sheet adhered to the film. Further, Japanese Laid-open Patent Application (Kokai) No. 2-107488 discloses a stencil comprising a thermoplastic film and a non-woven fabric mainly comprising synthetic fibers, which is adhered to the thermoplastic film.
However, these stencils are not necessarily satisfactory in the clarity of printed image. Although there may be various reasons therefor, one of the major causes is the so called white spots (the phenomenon that white defects are formed in the area painted in black). One of the causes of this phenomenon is that even when the film constituting the stencil is melted to form through openings, if the adhesive adhering the film with the support exists in the opened area, the permeation of the printing ink is inhibited by the adhesive and the points which constitute an image line on a printing paper cannot be formed.
Thus, in order to promote the printing quality and clarity of printed image, it is demanded to make the amount of the used adhesive as small as possible.
In response to this demand, various proposals have been made. For example, Japanese Laid-open Patent Application (Kokai) No. 58-147396 discloses a stencil comprising a net-like adhesive layer between a porous tissue paper and a synthetic resin film; and Japanese Laid-open Patent Application (Kokai) No. 4-232790 discloses a stencil in which the area of the adhesive is set within a specific range. However, by any of these methods, satisfactory results have not been obtained.
Further, the adhesives per se which are currently used also have problems. For example, acrylic resin-based adhesives and vinyl acetate resin-based adhesives have poor ink resistance because these adhesives are softened, swelled or dissolved in the printing ink. Curable adhesives have a drawback in that non-cured materials are likely to be formed, which are likely to be attached to the thermal head during processing. Chlorinated resin-based adhesives have a drawback in that toxic chlorine gas is liberated to the thermal head during processing.
Thus, a heat-sensitive mimeograph stencil which does not employ an adhesive at all is now desired.
To overcome these problems, Japanese Laid-open Patent Application (Kokai) No. 4-212891 proposes to form a heat-sensitive mimeograph stencil comprising a thermoplastic resin film and synthetic fibers scattered on one surface of the thermoplastic film, which are bonded to the film by thermocompression. However, with this method, if the adhesion between the resin film and the fiber layer is insufficient and so the peeling strength is small, the fiber layer is peeled off during transportation of the film, and the film is wrinkled or broken. Further, if fibers are bonded by a binder, the fibers are adhered to a heat roll so that films cannot be stably formed.
On the other hand, although Japanese Laid-open Patent Application (Kokai) Nos. 48-23865 and 49-34985 disclose thermally adhering a polyester film and a non-woven fabric and then subjecting the composite film to co-stretching, the composite film is not used as a heat-sensitive mimeograph stencil. Therefore, it is not disclosed that an excellent heat-sensitive mimeograph stencil is attained when the peeling strength is within a specific range.